Bogged by delays for six years, Adani may pull out from the proposed $21.5 billion rail and mine project in Australia amid a series of legal challenges from environmental groups against the Indian energy giant's plans to build one of the world's largest coal mines.

Stating that he was "disappointed", the company's founder and chairman Gautam Adani told 'The Australian' newspaper that the "pit to plug" project was yet to receive the green light after six years of environmental assessments and court battles.

"You can't continue just holding. I have been really disappointed that things have got too delayed," Adani said.

Adani said he hoped the court challenges to Australia's largest proposed coalmine would be finalised early next year.

However, with one court case yet to be heard in the Federal Court, and at least two groups threatening High Court action, Adani warned he could not wait indefinitely.

Adani said that he was already scouting alternatives to feed his power stations in India.

Confirming he had met Australian Prime Minister Malcom Turnbull last December to seek to deliver greater certainty on such projects, Adani said, "We were suggesting how to bring in the certainty of the timing."

"We were asking how we get certainty of the time schedules...that is the most important for us in committing all of our resources."

"It's just covering up the real fact that what is damaging the reef is an increase in the temperature of the seas through climate change," he said.

Another new Federal Court challenge to the mining lease for Carmichael, issued by the Palaszczuk Labour government, will be heard this year.

Adani said he originally believed the approvals process would take two to three years and that he has already spent $3 billion buying the tenements and the Abbot Point port lease.

The company is still exploring the financing issue of the project.

Adani said if there were no more unexpected delays, he had confidence that the project would get financing and "still be competitive" against other alternative sources of coal in India and Indonesia.

Adani Australia chief executive Jeyakumar Janakaraj said the co-ordinated campaign by anti-coal activists to block the mine had damaged Australia's international reputation. He said the business community in India had expressed concern about future investment in Australia.

"I think it has already turned off a lot of switches. I am not saying it is going to be permanent, but there has been damage."

Turnbull's office did not comment on Saturday about Adani's call for greater certainty to the approvals process. But the government has argued that all commonwealth approvals are in place and there are no remaining federal obstacles to the project proceeding.

Adani's plan to coal mines in Australia has been hampered time and again. A federal court in August last year had revoked the original approval due to environmental concerns.

In October last year, the project got a new lease of life after the Australian government gave its re-approval.

Australia's Queensland state government in April gave Adani permission to mine coal reserves estimated at 11 billion tonnes and to build roads, workshops, power lines and pipelines associated with the mine.

Environmentalists are fighting the approvals, saying the project will jeopardise the state's future and destroy national treasures like the Great Barrier Reef.